The invention relates to an attachment for the machining by stock removal of parts to be turned, particularly such which have surface longitudinal profiles which differ from the exact circular shape and may furthermore be irregular, with the aid of a tool whose in-feed movements are brought about by an electrically controlled servo-motor, whose follow-up time is very short, so that the tool nose can follow the pattern of the desired non-circular shapes without any limitation of the cutting speeds which are usual in turning. The use of an electrically controlled servo-motor is advantageous inasmuch as the templates and master pieces which are required on heretofore known non-circular turning lathes and from which the tool in-feed movements are traced by mechanical tracing through a control lever, can be entirely dispensed with. With the aid of known electronic data processing equipment, it is possible to store in digital or analogue form the desired three-dimensional shape of the part to be turned and to process it in such a way that the electric voltage which is analogous to the distance between the turning centre and the workpiece tip required at the respective workpiece location can be fed at the correct moment as a control signal to the electrically controlled servo-motor in synchronism with the working movements of the machine tool.
Such machine tool including controlled servo motor is known from German Pat. No. 12 68 936, wherein the servo-motor is formed by a magnetostrictive body which is surrounded by a coil through which current flows. Although working inaccuracies caused by play and wear influences, such as occur on devices which follow rotating master cams by means of a lever system for transmission to the part to be turned, are avoided on this known construction, the main disadvantage thereof consists in the fact that the controlling element length changes attainable by magnetostriction are very small in relation to the original control element length so that, in order to bring about sufficiently large in-feed movements, it is necessary to use very long magnetostrictive bodies, which are consequently difficult to accommodate, or to increase the control movement to the practically required value through mechanical transmission members, such as levers and the like, which entails a loss in accuracy.
The object underlying the invention is to avoid this disadvantage and to provide an attachment which requires little space and allows nevertheless a rapidly responding and exact in-feed movement of the tool.